Knowledge of wellbore placement and surveying is useful for the development of subsurface oil & gas deposits. Accurate knowledge of the position of a wellbore at a measured depth, including inclination and azimuth, may be used to attain the geometric target location of, for example, an oil bearing formation of interest. Additionally, directional borehole drilling typically relies on one or more directional devices such as bent subs and rotary steering systems to direct the course of the wellbore. The angle between the reference direction of the directional device and an external reference direction is referred to as the toolface angle, and determines the direction of deviation of the wellbore. Directional drilling proceeds through comparing the placement of the borehole with the desired path, and selecting a toolface angle and other drilling parameters to advance the borehole and correct it towards the planned path. Measurement of toolface thus may be a component for borehole steering and placement.
The measurement of inclination and azimuth of the wellbore may be used in surveying operations. Inclination is the angle between the longitudinal axis of a wellbore or a drill string or other downhole tool positioned in a wellbore and the gravity vector, and azimuth is the angle between a horizontal projection of the longitudinal axis and north, whether measured by a magnetometer (magnetic north) or by a gyro (true north).
One method of determining the orientation and position of a downhole tool with respect to the Earth spin vector is to take a gyro survey, referred to herein as a gyrocompass, to determine a gyro toolface, inclination, and azimuth. The gyrocompass utilizes one or more gyroscopic sensors, referred to herein as gyros to detect the Earth's rotation and determine the direction to true north from the downhole tool, the reference direction for a gyro toolface and azimuth. However, at high inclination, i.e. where the downhole tool is nearly horizontal with respect to gravity, a single-axis gyro substantially orthogonal to the downhole tool may be unable to determine true north to sufficient accuracy. Additionally, errors in gyro readings caused by, for example and without limitation, bias errors or mass unbalance, may be undetected and induce error in the determination of true north.
The determination of orientation, position, inclination, and azimuth of the downhole tool may include determining a gravity toolface or magnetic toolface by using one or more accelerometers or magnetometers respectively. Accelerometers may be used to detect the local gravity field, typically dominated by the Earth's gravity, to determine the direction to the center of the Earth. This direction may be used as the reference direction for a gravity toolface. Magnetometers may similarly be used to detect the local magnetic field, typically dominated by the Earth's magnetic field, to determine the direction to magnetic north. This direction may be used as the reference direction for a magnetic toolface. However, errors in the sensor readings, such as offset or drift, may be undetected and induce error in the determination of toolface.
Typically, gravity toolface is utilized except where the inclination is very low, such as, for example and without limitation, 5° or less. In low inclinations, cross-axial accelerometers may measure only a small gravity signal. At low inclinations, gyro or magnetic toolface is traditionally utilized for orienting toward the target drilling direction due to the large cross-axial signal of the Earth's spin vector or magnetic field.